Ball bearings for automotive applications may be found in both low speed applications, such as above the spring seat in a McPherson type strut, or in high speed applications, such as wheel bearings. While low speed bearings can tolerate a ball row with no cage to keep the balls from colliding, high speed ball bearings require some kind of cage type separator. In the case of a high speed bearing where the load requirements do not require a very close ball spacing or packing, there is significant room between adjacent ball pairs. In such a case, it is relatively easy to provide a molded plastic bearing cage. Structural elements of the plastic cage, such as relatively thick fingers or cross bars, pass between the balls in the room available. Examples of these may be seen in U.S. Pat. No. 4,330,160, which shows a closed ball pocket that completely encircles a ball, and co assigned U.S. Pat. No. 4,938,613, which shows an open pocket that securely retains, but does not completely encircle, a ball. Each patent also discloses a design that can be by pass molded, sometimes called axial draw molding. This is a molding method in which only two, axially parting molds create all molded surfaces. In U.S. Pat. No. 4,330,160, the moldability of the design has the drawback of creating ball contact surfaces in the pocket that would not securely hold the ball apart from the bearing assembly, which could limit ease of assembly.
In the case of bearings that require a very close ball packing, a different retainer design is needed, one that does not occupy space between adjacent balls. Typically, such cages, which may be referred to as open profile cages, are made from stamped and folded metal. An example may be seen in U.S. Pat. No. 1,203,928. In an open profile cage, the space between the balls is left open by separating the balls with fingers that extend out axially from an annular spine to wrap partially around the balls, keeping them apart, but which do not block the closest spacing between the balls. Stated differently, the closest spacing between the balls is along the chord length of the balls, that is, a straight line running from one ball center to the adjacent ball center. In a true, open profile cage, no solid part of the cage is encountered moving along the chord length of the balls, allowing them to be spaced as closely as possible. 0f course, as a practical matter, there will have to be some space between the balls to prevent ball to ball rubbing, the cage structure is not a limiting factor.
It is a relatively simple matter to make an open profile cage from metal, as the high strength of metal allows for very thin fingers and spines, and the metal folding and bending technology is well developed. It would not be possible to simply replicate such old metal designs in plastic, however. The much weaker plastic material would require much thicker cross sections for the spine and fingers, and the folded metal designs often are too complex to be molded by the axial draw method described. U.S. Pat. No. 4,702,627 attempts to provide a molded plastic cage that allows for a close packed ball row. However, as seen in FIG. 8 of the patent, which shows a cross section of the cage as seen moving along the chord length, a solid web of material is encountered. Therefore, the cage cannot be considered a true open profile design. It would inherently limit the closeness of the ball packing, especially in the thick sections needed with plastic. Also, the design shown would clearly not be moldable by simply axially parting two molds, at least not without stressing certain elements of the cage. As seen in 2 of the patent, the ball pocket is provided, for each ball, by two fingers 9 that wrap or hug one side of the ball, and a semi spherical seat 8 that contacts the opposite side of the ball. The fingers 9 lie partially over the seat 8, so the mold element that created the seat 8 would have to be bend the fingers 9 out of the way as the molds parted. This is feasible, since the fingers 9 have room to flex, but it is not true, stress free axial draw molding.